The respiratory burst oxidase (RBO) is the enzyme responsible for superoxide production by phagocytes and B lymphocytes. It is dormant in resting cells but comes to life when the cells are activated. Among the components of the RBO is a 47K (formerly 48K) protein known as p47phox that becomes phosphorylated when RBO-containing cells are activated. The aims of the present work are to identify the sites of phosphorylation in p47phox and elucidate their relationship to the activity of the RBO; to identify proteins with which p47phox interacts and understand these interactions at a molecular level; and to produce large quantities of p47phox for structure determination by X- ray crystallography.